Electric oscillator



Sept. 19, 1939. H. H. SCOTT 2,173,427

ELECTRIC OSCILLATOR Filed Aug. 30, 1937 5 Sheets-Sheet 2 (1L I M8 HERMON HOSMER SCOTT INVENTOR BY Ma aw ATTORNEY Sept. 19, 1939. H, H, SCOTT 2,173,427

ELECTRIC 05 C ILLATOR I Filed Aug. 30, 1937 5 Sh9ats-Sheet 3 IST.ANPL|F'IER ZND- AMPLIFIER 5RD.AMPLIFIER ST AGE ST AGE ST AGE 5' r Y P Q C C O C OUTPUT O 0 O O O O REGENERATION W {D NETWORK A DEGENERATIONC 0' NETWORK O F'G'al FREQUENCY FREQUENCY HERMON HOSMER SCOTT INVENTOR ATTORN EY Sept. 19, 1939. i H. H. SCOTT 2,173,427

ELECTRIC OS CILLATOR Filed Aug. 50, 19s? 5 Sheets-Sheet 4 HERMON HOSMER- SCOTT INVENTOR ATTORN EY FIG. 9.1

Sept. 19, 1939.

H. H. scoT ELECTRIC OSCILLATOR Filed Aug. 30, 1937 '5 Sheets-Sheet 5 O m m m e l.

M f .A Y C N M U Q E m w 3 x9 36m 52m 5": m rE$zmmun uzifizmswz REGENERATIVE NETWORK DEGE'NERATIVE NETWORK BRIOQEDT HERMON HO SMER SCOTT INVENTOR I BY AW FIG. ll.

' ATTORNEY Patented Sept. 19, 1939 UNITED STATES ELECTRIC OSCILLATOR Hermon Hosmer Scott, Somerville, Mass, as-

signor to General Radio Company, Cambridge, Mass, a corporation of Massachusetts Application August 30, 1937, Serial No. 161,622

34 Claims.

, The present invention relates to electric oscillators, and more particularly to oscillators comprising amplifiers, such as vacuum tubes.

Various devices are in use or have been proposed 6 for obtaining an electrical circuit which is selfoscillatory. The well-known Colpitts and Hart ley circuits are typical examples, and many variations or modifications of these or similar circuits are widely used. Such circuits can usually be adjusted to provide an output wave-form which is reasonably close to a'sinusoid. In general, however, these circuits are all characterized by the fact that inductance is a necessary part of one or more of the circuit elements in order to obtain the sinusoidal wave-form.

Oscillators employing resistances and capacitances only, but no inductances, are also used. Multivibrators and cathode-ray-oscillograph timing oscillators, or sweep circuits are typical illustrations. A general characteristic of such circuits, however, is a saw-tooth, or irregular, waveform departing widely from a theoretical sinusoid and containing a large number of harmonics of the fundamental frequency, said harmonics being of appreciably higher amplitude than would be considered satisfactory for a sinusoidal oscillator.

A chief object of this invention is to provide a simple and satisfactory means of obtaining an voltage, but which does not contain any inductances as necessary circuit elements in order to obtain this sinusoidal wave-form. Other importantfeatures of the invention are the ease with which the frequency of the oscillations may be controlled and the practical suppression of all harmonics, as well as extraneous voltages, hum, etc., picked up by or generated by the circuit elements, vacuum tubes, etc.

Other and further objects will be explained hereinafter and will be particularly pointed out in the appended claims.

The invention will now be explained in connection with the accompanying drawings, in which Fig. l is a diagrammatic simplified view of circuits and apparatus arranged and constructed according to the present invention; Fig. 2 is a similar view of a modification; Fig. 3 is a diagrammatic view illustrating one form of degeneration network; Figs. 4 and 5 are similar views illustrating other forms of degeneration network; Fig. 6 is a view similar to Fig. 1 of a further modification; Fig. 7 is a diagram illustrating a response curve that is representative of many prior-art types 55 of networks, including the hereinafter described oscillator which will provide a sinusoidal output I degenerative network; Fig. 8 is a similar diagram illustrating a response curve in accordance with the present invention; Fig, 9 is a diagram illustrating a preferred embodiment of the invention; Fig. 10 is a view similar to Fig. '7 of a further response curve; and Fig. 11 is a view of a further modification.

The response curve shown in Fig. 7, showing the relation between the output voltage and the frequency, is representative of many types of networks, including some combinations of resistance, capacitance and inductance, or mechanical, piezoelectric or magnetostriction devices. The characteristic feature of a curve of this type is that a single frequency or band of frequencies is cancelled out practically entirely, while a reasonable degree of transmission is obtained at other frequencies.v In other words, such a device is a form of band-elimination filter.

Such a characteristic can be obtained in a network using only resistances and capacitors such as, for instance, the Wheatstone bridge, including the Wien bridge, and a parallel-T or bridged-T network. The Wien bridge, for example, illustrated in Fig. 3, is provided with resistors 49, 50, 5i and 52 in its respective arms, and capacitances 54 and 55 respectively in series with and in shunt to the respective resistors 52 and lit. The resistors 49 and 50 are preferably fixed and the resistors 5| and 52 are variable. The parallel-T network consists, as its name implies, of two T networks connected in parallel, as illus-. trated in Fig. 4, comprising only resistances 3, 4 and 5 and condensers 6, 'l and 8, since this is a convenient form of such a network, but inductances may also be included if desired. The particular T network shown in Fig. 4 has certain desirable characteristics which will be discussed later.

A bridged-T network may comprise only a single -T network as, for instance, the circuit elements 5, 6 and l of Fig. 5, but with another impedance connected directly in shunt with the two series-arms 6 and I. This is illustrated in Fig. 5, the impedance connected between the opposite terminals of capacitances 6 and 1 being represented by the coil I66. It will be understood that a the bridged-T, like the parallel-T, network is not restricted to resistors and capacitors. The elements of both networks may comprise any combination of impedances, including inductors.

The desired characteristics for a selective circuit in accordance with the present invention, as previously outlined, may be practically the inverse of that shown in Fig. 7; that is, the circuit should respond to a single frequency or a narrow band of frequencies and should attenuate or therefore, a circuit having the characteristic shown in Fig. '7 may be combined with suitable auxiliary apparatus so that the net over-all characteristic shall be as shown in Fig. 8. The inverse of the usual characteristic of the curve is thus obtained, a band-suppression networkbeing emnloyed to nroduce a band-pass system.

This may be effected in any desired way, as, for example, is diagrammatically illustrated in Fig. 1. The amplifier I there diagrammatically illustrated may be of any desired type, providing an amount of gain, the importance of which will be discussed hereinafter. The input terminals of the system, shown as 3 and 4, are normally connected together through an impedance, or directly. For instance, if the system is functioning as a band-pass or selective amplifier a source of energy may be connected to terminals 3 and 4. If the system is functioning as an oscillator, as hereinafter set forth, terminals 3 and 4 may be connected together directly, and sufiicient regeneration provided in the circuit to cause oscillation to take place. The amplifier is designated by I in Fig. 2. It is shown provided with input terminals I02 and IM, connected to its input circuit, and output terminals I 06 and I 08, connected to its output circuit. In Fig. 1, the terminals I02 and I04 are shown connected by conductors I I0 and H2 to the output circuit of a network 2, as hereinafter more fully described. The terminals I06 and I08 may similarly be connected by conductors I I4 and H0 to the input circuit of the network 2. The conductors H4 and H6 may also be connected to terminals 5 and 6, which thus constitute the output terminals of the system. These output terminal are represented in Fig. 2 at I0, II.

The said network 2 is connected so as to feed back energy degeneratively from the output cir-. cuit of-the amplifier to its input circuit. To this end, the input of the feed-back network 2 is shown connected by conductors H0 and I20 to the output circuit of the amplifier I, and the output of the network is connected to the input circuit of the amplifier.

The transmission characteristics of the feedback network 2 are similar to those shown in Fig. 7, so as to provide maximum attenuation or zero transmission or coupling at that frequency or frequencies where it is desired that the systern as a whole shall provide maximum transmission. At other frequencies, which the amplifier would normally pass, the network provides a certain amount of transmission and the voltage obtained from the network is fed back to the amplifier input, so that it has a phase reversal with respect to the voltage originally between the terminals I02 and I04, thus reducing the efiective gain of the amplifier. By phase reversal, it is intended to denote a shifting of the phase between 90 degrees and 270 degrees. At the frequency or frequencies where the degenerative network provides zero or very low transmission or coupling, the net effective gain of the amplifier is at its maximum or near its maximum, respectively. The resultis that the transmission characteristic of the system as a whole has a general characteristic as shown in Fig. 8. It will be seen that the difference in transmission of the system between the frequencies of maximum and minimum transmission, which characteristic may be defined as the maximum suppression of the system, depends upon. both the gain of the amplifier and the transmission charac eristics of the degeneration network. The effect of shifting the amplifier gain is to change appreciably the transmission at those frequencies where the transmission is greatest with only a comparatively minor change in the transmission where it is at a minimum,

thus modifying the selectivity curve and changing the maximum suppression.

In accordance with the present invention, therefore, selectivity is obtained in an amplifier with the aid of a network which may, but does not necessarily, contain resistances and capacitances only, but without introducing loss in the normal transmission of the system, at the frequencies of maximum transmission.

It is well known that an electrical amplifier can be made to produce sustained oscillations if the output circuit is properly coupled to the input circuit. If the amplifier is sharply selective to a single frequencythat is, if its response is at a maximum at a single frequency-it will tend. to oscillate at that frequency.

In Fig. 1, the network 2 has such characteristics as to produce degeneration, in that the output of the amplifier is fed back to the input with such a phase shift that the effective gain of the system is reduced. This degeneration network, however, is designed to have a null point at a certain predetermined frequency-that is, its transmission at this frequency is practically zero, as described above in connection with Fig. 1,and, accordingly, at this frequency practically no degenerative voltage is applied to the input of the amplifier. Accordingly, at said frequency, the system as a whole provides practically the complete gain of the amplifier. Since the degeneration network may consist of resistances and capacitances only, as will be described later, the system as a whole functions as a selective amplifier, as described above in connection with Fig. 8, although no inductances need be employed.

By varying the elements of the'degeneration network it is possible to vary the frequency to which the system is selective. In common with other selective amplifiers this amplifier can be made to oscillate at its selective frequency if a sufllcient amount of regenerative coupling exists between the output and the input. Fig. 2 shows such an arrangement, consisting of the amplifier I, the degeneration network 8, which two units comprise the selective amplifier, plus a regeneration network 9. The characteristics of this network are such that a regenerative voltage is fed from the output of the amplifier back to the input at the frequency where the degenerative network 8 provides no degenerative voltage. The characteristics of the regenerative network may be selective-that is, it may provide regenerative feed-back only at this frequencyor the network may be substantially aperiodic, so that regenerative feed-back takes place through this network over a wide range of frequencies. An obviously necessary condition is that the regenerative feed-back shall be small compared to the maximum degenerative feedback. In actual practice, best operating conditions are generally found to be when the regenerative feed-back is only slightly larger than the amount necessary to provide sustained oscillations.- Consequently, the degenerative feed-back at frequencies a reasonable distance from the oscillation frequency is so large compared to the regenerative feed-back that suppression of harmonics, internal amplifier noise, etc., is obtained. To this end, the amplifier output circuit should be degeneratively coupled provision for such phase reversal does not have to be made in the degeneration network. Such an amplifieris easily obtained by employing three vacuum-tube stages. Afurther refinement is the use of a direct-coupled amplifier in which the phase 'shift, even at very low frequencies, is not of serious importance, thus simplifying the deto its input circuit at frequencies located above\ sign of the degenerative and regenerative the oscillatory. frequency by one octave or more, if harmonic suppression is to take place;

The two networks 8 and 9 may, of course, be considered as a single network whichprovides regenerative feed-back at the frequencies where oscillations are desired and degenerative feedback at other frequencies. The single network 2 of Fig. 1 may be such a network, or it may be replaced by such a network. Not only may the two networks 8 and 9 be considered as one, but one of the simplest ways of constructing an' oscillator of this type includes the use of a single physical network for the purpose, as will be described later on in this specification.

A characteristic common to practically all vacuum-tube oscillators is that regeneration takes place at the-oscillation frequency. An important feature of the present invention, on the other hand, is that degeneration takes place at other frequencies than the oscillation frequency, being suiflciently great at twice the oscillation frequency and higher frequencies so that harmonics of the oscillation frequency are effectively suppressed. Many networks which may conveniently be used for the purpose provide also degeneration at lower frequencies, so that hum generated or picked up by the amplifier or other low-frequency disturbances are also suppressed.-

The degenerative action isalso. effective in lowering-.the general noise level obtained from the A further feature of this invention is that a 'merely varying or switching resistances is a distinct advantage from the standpoints of convenience, cost, and ease of manufacturing. The amplifier may be of any type operating on the vacuum-tube, electro-mechanical, or other principles. Since the harmonic suppression dependsupon the degenerative action, however, the harmonic distortion in the output wave-form will decrease as the amplifier gain is increased, providing the degeneration network remains fixed and the regeneration network is adjusted, as previously described, to provide sufilcient but not excessive regeneration. Experience has shown that an oscillator of this type comprising a three-stage amplifier using commercially available vacuum tubes will provide nearly the full rated output of the last tube in the amplifier with a distortion of a few tenths of a per cent. Obviously, such a system compares favorably with more conventional types of oscillators employing inductances.

In actual practice, it is frequently desirable to use an amplifier in which the phase between the input and output circuits is reversed, so that primary winding I38 of which systems. v Regarding the degeneration network, this may be practically any type of network which pro-'- vides a reasonably sharp null balance. Since there are a great number of filter and bridge circuits which provide zero or very low transmission at a single frequency or over a band of frequencies, it is obvious that the invention covers a large number of possible circuit arangements. Figs. 3 and 4 show two systems employing common types of resistance-capacitance networks for the inverse feed-back systems, and a third type is shown in Fig. 5. InFIg. 3,-the inverse feed-back system is illustrated as comprising the before-mentioned Wien bridge. Circuits of this type are widely used for measuring frequency, in which application the suppression of a single frequency or narrow is important. In this use of the Wien bridge, however, the combination with the amplifier and associated apparatus provides an over-all response characteristic which is at a maximum at that frequency to which the bridge is balanced. In a circuit of this type, the response characteristics are determined by the relative values of the resistances and the condensersl The characteristlcs of the network may be varied by varying all or some of these components.

As illustrated in Fig. 3, the two opposite vertices I22 and I24 of the bridge may be respectively connected to the conductors H0 and H2. The output circuit of the Wien bridge is thus connected to the input circuit of the amplifier. The vertices I22 and I24 may be directly connected to the amplifier input circuit by the conductors H2 and H0, or a transformer 2I0 may be interposed. The other two opposite vertices I and I are similarly shown respectively connected by conductors I32 and I34 to the secondary winding I36 of a transformer the is connected by the conductors H4 and H6 to the amplifier output terminals I 06 and I08, but these vertices I28 and I30 may be directly connected to the terminals I00 and I08 without the aid of the transformer 40. The output circuit of the amplifier is thus connected to the input circuit of the Wien band of frequencies bridge.

The coupling of the degenerative coupling means,'which, in this case, is the Wien bridge, is substantially zero at the predetermined single frequency in question, as is true with all networks having an exact null or zero at balance. The coupling transformers 40 and 2", if used, merely provide the proper balanced-to-ground circuits for the bridge.

Fig. 4 represents an improvement over the circuit shown in Fig. 3 in that the degeneration network consists of a three-terminal network having one terminal I40 common to both its input.

and its output circuits. The terminal I40 is shown connected by a conductor I42 to the terminal I08 and by a conductor I44 to the terminal I04. One of the other two terminals of the network is shown at I46, connected'by a conductor I40 to the terminal I 06. The third terminal of the network is shown at I50, connected by a conductor I52, to the terminal I02. The particular three-terminal network shown is the previously described parallel-T network, having a characteristic similar to the Wien bridge of Fig. 3, and the frequency of zero transmission may be varied by shifting the resistances 3, 4 and 5, or the condensers 8, i and 8 or all six components.

There are several advantages of a system of this type, which should be noted. In the first place, if the degeneration or inverse-feed-back network provides complete suppression at a certain frequency, the amplifier gain is normal at that frequency and not affected by the network.

Accordingly, if an adjustable network is used so that its zero transmission frequency is adjustable and if the network is so designed and constructed that practicaly complete suppression is obtained at any frequency to which it is ad-' justed, the sensitivity of the amplifier to that frequency will be the same as the sensitivity of the amplifier to the same frequency when the network is not used. It is, accordingly, a relatively simple matter to obtain practically constant sensitivity over a wide frequency range by merely using an amplifier with practically constant gain over. that range and a feed-back network which gives practically complete suppression at the passed frequency. Of course, if the amplifier characteristic is not fiat over a wide range, or if the feed-back network does not give complete suppression, one of these characteristics may be made to balance the other. It should also be noted that since the feed-back network operates directly into a grid circuit, as

illustrated in Fig. 9, or into a high-impedance circuit, and may consist itself of high impedances, no appreciable amount of power need be drawn from the amplifier output by the feed-back system.

According to the preferred embodiment of the invention illustrated in Fig. 9, the amplifier is of the three-stage direct-coupled type, the stages being represented by vacuum tubes I2, I4 and I6, each comprising a cathode I55, 9. grid I54 and an anodeor plate I58. The input circuit of the amplifier may be traced from the conductor I52,

connected to the output circuit of the parallel-T network, to the terminal I82, and thence to the grid I54 of the tube I2; and from the grid I54 to the filament or cathode I55, a resistor 22 and a condenser 23, in parallel, to the terminal I84; and by way of the conductor I44 to the output circuit of the parallel-T network. The tube I2 is provided with an output circuit extending from its plate or anode I58, by way of a conductor I60, and through a resistor I3, to a plate battery 24, and thence, by way of a conductor I52, to the filament I54. The input circuit of the second stage I4 of the amplifier is shown connected across the resistor I3, and. its output circuit is provided with a similar resistor I5 across which the input circuit of the third stage I5 is connected. The output circuit of the amplifier extends from the plate or anode I58 of the tube I8, by way of a conductor I64, to the terminal I; and through a resistor I! to the battery 24, and the terminal I88. Each grid I54 of each tube except the first is galvanically or directly connected to the anode of the preceding tube. This output circuit is coupled to the input circuit of the parallel-T network, as before described.

The connection from the cathode I55 of the tube I2 to the resistor 22 is shown as variable, thus to render it possible to vary the gain of the amplifier, thus to adjust the width of the band An amplifier of this type is not necessary, but

the particular arrangement shown is used since the phase shift at low frequencies between the grid I54 of the first tube I2 and the plate I58 of the last tube I8 is always between 180 and 270 degrees. This simplifies the design of the feedback circuit. Any other type of amplifier can also be used, however, it being merely necessary to keep the total phase shift through the amplifier and the feed-back circuit such that a serious amount of regeneration does not take place. That is, for any frequency for which the aniplifier gain is greater than theattenuation in the feed-back network the total phase shift should be between and 2'70 degrees in order to provide proper degeneration, excepting, of course, in the neighborhood of the oscillation frequency. The term "phase reversal will be employed to indicate such a phase shift of the network. For these reasons, an amplifier comprising an odd number of stages is frequently desirable and the direct-coupling feature prevents undesirable phase shifts, even at very low frequencies. I

The amplifier shown is battery-operated at 24, but, obviously, any suitable power-supply equipment could be used. The heaters or filaments I55 of the tubes must, of course, be operated from suitable batteries or transformers.

The parallel-T network enclosed within the dotted line marked 2I2 comprises a feed-back circuit of the type shown in Fig. 4. The particular arrangement shows the resistances 2, 4 and 5' variable, and ganged together for operation by a single control, as is illustrated more particularly in the said copending application. Theoutput of the network 2I2 is fed back into the amplifier I by means of the conductors I 44 and I52. The over-all transmission characteristic of such a system is shown in Fig. 8.

The parallel-T network, including resistances and capacitances only, is shown in the preferred embodiment of the invention of Fig. 9, because this particular type of network is extremely useful at audio frequencies. One condition of balancefor such a network is obtained when capacitance 6 equals capacitance I, each equaling one-half of capacitance 8, when resistance 3 equals resistance 4, each resistance 3 and 4 equaling two times resistance 5, and when the frequency is such that the reactance of capacitance 8 is equal in magnitude to resistance 3. This condition of balance for the parallel-T network, as shown in Fig. 9, results in very convenient values for the circuit elements in this network. 'obviously, this combination of values lends itself readily to simultaneous operation of the resistances or the condensers by means of a common control. A convenient method for varying such a network over wide ranges at lower frequencies is to employ three variable resistances which are operated by a common control, as illustrated in the said copending application.

By switching various sizes of condensers into the 1 circuit as components 6, I and 8, the frequency to which the system is selective may be varied in steps. At higher frequencies, it is sometimes desirable to use variable condensers, which may or may not be connected to a common shaft, and

- arrangement at lower frequencies is continuously to adjust the three resistances 3, 4 and simultaneously by means of a single control arm, and to shift by steps the condensers 6, l and 8 to other condensers, respectively. by means of a multi-pole switch. At higher frequencies, it may be simpler to use continuously variable condensers 6, I and 8 operated by a single control arm and to switch by steps the resistances 3, I and 5 to other resistances, respectively.

One advantage of such a network is that the frequency maybe varied by varying the three resistances orthe three. condensers, or both. For frequency ranges where it is feasible to vary the resistors, this provides an extremely satisfactory and economical means of control. Such a network may also be modified so that, instead of balancingsharply to an exact null, it provides a small amount of regeneration at essentially the same frequency, thus combining the functions of the regenerative and degenerative networks shown in Fig. 2.

In Fig. 1, the single network may provide sufflcient regeneration in the neighborhood of the oscillation frequency to provide sustained oscillations, but suflicient degeneration at other frequencies for satisfactory suppression of harmonies, noise, etc. For instance, if the network shown in-Fig. 4 is used in Fig. 1 for the feedback network, its circuit elements may be so selected as to provide the proper characteristics. A further illustration is afforded by Fig. 9 if the adjustable resistor 2l3 and the condenser Ill in series therewith, as-enclosed in the dotted line marked 2l6, are omitted. This dotted line 2i represents a network that is used to provide regenerative feed-back.

It is illustrated in this case as comprising the elements M3 and 2, with the element 2" in the form of a potentiometer for varying the amount of the feed-back, and with the element 2" as a condenser for keeping the plate voltage of the tube ll off of the grid of tube II. The elements H3 and 2 represent only one of many possible combinations which could be used as the regenerative feedback network 2l6. Another possible arrangement is shown in Fig. 11, where the regenerative network 2 is shown consisting of a potentiometer 265 with a slider 26! and a blocking condenser 268. This network may also include a transformer or other types of circuit elements. The networks 9 in Fig. 2 and II! in Fig. 6 fulfill the same functions as the networks 2l6 of Fig. 9 and IN of Fig. 11.

An oscillatory condition may be obtained without a separate regeneration network, for instance, if the resistor 5 is reduced somewhat below the calculated exact value for a perfect null point. A similar effect is obtained by similarly varying the resistors 3 or I, or condensers G, I or 8, or by varying any of the other components in the proper direction. Fig. 10 shows the variation in the feed-back voltage obtained from a network of this type. That portion of the curve above the line 350 indicates degenerative feedback, while that below the line 350 indicates regenerative feedback. The step from Fig. '7 to Fig. 10 is obtained by varying the elements, as described above. This result can be obtained, for instance, with the circuits shown in Figs. 3, 4 and 5, by varying one or more circuit elements in'the proper direction away from the values required for exact null balance.

If a separate regenerative feed-back network is employed, there are many possible arrangements. For instance, a transformer may be used to feed suflicient energy from the output of any amplifying stage to the input of the same or a previous stage and in the proper phase to provide sufficient regeneration. If the amplifier comprises three or more stages, a feed-back voltage may be coupled from the output of any of the odd-numbered stages except the first into the input of a preceding even-numbered stage, as shown in Fig. 6, by means of-a transformer, voltage divider, or other coupling device, or, if

the amplifier comprises two or more stages, the

feed-back voltage may be coupled from the output of any of the even-numberedstages into the input of a preceding odd-numbered stage. Fig. 9 shows this latter arrangement, the network 2|6 being connected between the output circuit of the second stage It and the input circuit of the first stage I3. As previously pointed out. it is not necessary that the regenerative network be selective as to frequency, but merely that the regeneration voltage, except in the neighborhood of the oscillation frequency, shall be small compared to the degenerative voltage, so that the selectivity characteristic of the combined amplifier and degeneration networks, which characteristics provide the means for obtaining an excellent sinusoidal output wave-form, are not seriously impaired. Of course, a selective regenerative circuit which transmits only in the neighborhood of the oscillation frequency may be employed, but it is not necessary for satisfactory operation of the oscillator.

If the regenerative circuits are so designed that the regeneration is of suitabl magnitude over the range of frequencies which the oscillator is intended to cover, the frequency of the oscillator can, accordingly, be adjusted by merely adjusting the degeneration network. Such a network may, for many applications, consist of resistances and capacitances only and variation is a relatively simple matter. The preferred arrangement for low-frequency use is to vary the frequency continuously by controlling the resistances in the de-.

generation network simultaneously by means of a single control and to vary the frequency in steps by switching various sizes of condensers into and out of the circuit, as illustrated and described in the said copending application. For higher frequencies, it is. sometimes more convenient to vary the condensers continuously and the resistances in steps, as is also disclosed therein. At high frequencies a bridged-T network, such as is illustrated in Fig. 5, is also sometimes easier to manufacture or to control than the parallel-T networks.

Fig. 11 shows a further modification of the circuit, in which only two amplifying stages are used, and the necessary phase reversal for degeneration purposes is obtained in the transformer 260. This diagram shows also another possible form of regeneration network 2H comprising a potentiometer 265 and a condenser 20.. The potentiometer 285 is connected in the output circuit of the tube It and the condenser 268 is connected between the grid I54 of the tube It and the slider I" of the potentiometer through the resistor Ii. This network I operates practically the same as the network iii of Fig. 9. For the sake of variety, the grid biases in this Fig. 11 for the grid I! of the respective tubes i2 and II are shown as separate batteries 189 and ly coupling the output circuit to the input circuit to feedback energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the input circuit and the voltage applied to the input circuit by the network and means for providing sufilcient regenerative coupling between the output circuit and the input circuit to cause selfoscillation totake place at the said predetermined frequency.

2. An oscillator having, in combination, an-

amplifier having an input circuit and an output circuit, a network having an input circuit and an.

output circuit and having a characteristic curve such that the network shall attenuate a predetermined frequency, means for coupling the network input circuit to the amplifier output circuit and the network output circuit to the amplifier input circuit to feed back energy from the amplifier output circuit to the amplifier input circuit through the network, the oscillator comprising means for efiecting a reversal of phase between the voltage in the amplifier input circuit and the voltage applied to the amplifier input circuit by the network output circuit, and means for providing sufilcient regenerative coupling between the amplifier output circuit and the amplifier input circuit to cause self-oscillation to take place at the said predetermined frequency.

3. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a three-terminal network having a characteristic curve such that the network shall attenuate a predetermined frequency, means connecting two of the terminals of the network to the output circuit and one of the said two terminals and the third terminal of the network to the input circuit to feed back energy from the out put circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage in such that the network shall attenuate a predetermined frequency, and means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to, the inputcircuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the input circuit and the voltage applied to the input circuit by the net-- work and means for regeneratively coupling the output circuit through the input circuit at the said predetermined frequency to cause selfoscillationtctake place.

5. An oscillator having, in combination, an

- amplifier having an input circuit and an output circuit, the amplifier comprising a plurality of amplifying stages each having an input circuit oscillator comprising means for efiecting a reversal of phase between the voltage of the amplifier input circuit and the voltage applied to the amplifier input circuit by the network, and means for coupling the output circuit of any oddnumbered stage except the first stage to the input circuit of any preceding even-numbered stage to provide sufilcient regeneration to cause self-oscillation to take place at the predetermined frequency.

6. An oscillator having, in combination, an amplifier. having an input circuit and an output circuit, the amplifier comprising a plurality of amplifying stages each havingan input circuit and an output circuit, a network having a characteristic curve such that the network shall attenuate a predetermined frequency, means for degeneratively coupling the amplifier output circuit to the amplifier input circuit to feed back energy from the amplifier output circuit to the amplifier input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the amplifier input circuit and the voltage applied to the amplifier input circuit by the network, and means for coupling the output circuit of any evennumbered stage to the input circuit of any preceding odd-numbered stage to provide sufiicient regeneration to cause self-oscillation to take place at the redetermined frequency.

7. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a. network having a characteristic curve such that the network shall attenuate a predetermined frequency, the network containing a plurality ofresistive impedances and a plurality of capacitive impedances, means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the input circuit and the voltage applied to the input circuit by the network, means for adjusting a plurality of the impedances, and means for providing sufiicient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency.

8. An oscillator comprising an amplifier having an input circuit and an output circuit, means for degeneratively coupling the output circuit to the input circuit, the said means having substantially minimum coupling at a predetermined frequency and comprising a plurality of resistors and capaci- 76 tors only, means for varying the resistors only to vary the frequency of the oscillations of the oscillator, means for reducing the transmission through the degenerative-coupling means to a; minimum at the said predetermined frequency, and means for providing sufilcient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency.

9. An. oscillator having, in combination, an amplifier having an input circuit and an output circuit, a. network having a characteristic curve such that the network shall attenuate a predetermined frequency, the network containing a plurality of resistive impedances and a plurality of capacitive impedances, means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the input circuit and the voltage supplied to the input circuit by the network, means for simultaneously adjusting a plurality of the impedances, and means for providing sufiicient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency.

10. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a network having a characteristic curve such that the network shall attenuate a predetermined frequency, the network containing a plurality of resistive impedances and a plurality of capacitive impedances, means for degeneratively coupling the output circuit to theinput circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the input circuit and the voltage applied to the input circuit by the network, means for adjusting a plurality of the impedances, means for simultaneously adjusting a plurality of the resistive impedances, and means for simultaneously adiusting a plurality of the capacitive impedances.

11. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a parallel-T network having a characteristic curve such that the network shall attenuate a predetermined frequency, means for degeneratively coupling the output circuit to the input-circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage in the amplifier input circuit and the voltage applied to the amplifier input circuit by the network, and means for providing sufficient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency.

12. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a parallel-T network having a characteristic curve such that the network shall attenuate a predetermined frequency, the network containinga plurality of resistive impedances and a plurality of capacitive impedances, means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network,

the-oscillator comprising means for effecting a reversal of phase between the voltage in the input circuit and the voltage applied to the input circuit by the network, means for providing sufficient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency, and means for adjusting one or more of the impedances.

13. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a bridged-T network having a characteristic curve such that the network shall atten-m pling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency.

14. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a bridged-T network having a characteristic curve such that the network shall attenuate a predetermined frequency, the network containing a plurality of resistive impedances and a plurality of capacitive impedances, means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage in the input circuit and the voltage applied to the input circuit by the network. means for providing sufficient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency, and means for adjusting one or more of the impedances.

15. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, the amplifier comprising an odd number of amplifying stages, a network having a characteristic curve such that the network shall attenuate a predetermined frequency, means for degeneratively coupling the output circuit to the input circuit to feed backenergy from the output circuit to the input circuit through the network, and means for providing sufiicient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency.

16. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, the amplifier comprising an odd number of amplifying stages, each stage containing a vacuum tube having a cathode, a grid'and a plate,- lnput andv output circuits for each' tube comprising, respectively, the grld and the cathode. and the plate and the cathode, of the respective tubes, each grid of each tube except the first being galvanically connected to the plate of the preceding tube, means for degeneratively coupling the amplifier output circuit to the amplifier input circuit, means for minimizing the said coupling action at the said predetermined frequency, and means for providing sumclent regenerative coupling between the amplifier output circuit and the amplifier input circuit to cause self-oscillation to take place at the said predetermined frequency.

17. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, the amplifier comprising an even number of stages, a network having a characteristic curve such that the network shall attenuate a predetermined frequency, means comprising a transformer for degeneratively coupling the output -circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network, and means for providing sufficient regenerative coupling between the output circuit and the input circuit to cause selfoscillation to take place at the said predetermined frequency.

18. An oscillator having, in combination an amplifier having an input circuit and an output circuit, a network having a characteristic curve such that the network shall attenuate a predetermined frequency, means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal ,of phase between the voltage of the input circuit and the voltage applied to the input circuit by the network, means for providing sumcient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency, and means for varying the attenuation of the voltage on the output circuit.

19. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a network having a characteristic curve such that the network shall attenuate a predetermined frequency, means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the input circuit and the voltage applied to the input circuit by the network, means for providing suincient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to'take place at the said predetermined frequency, and means for varying the gain of the amplifier to vary the wave form of the oscillations.

20. .An electrical circuit oscillatory at a predetermined frequency and comprising an amplifier having an input circuit and an output circuit, a single network for coupling the output circuit regeneratively to the input circuit at the DSC11139 tory frequency and for degeneratively coupling the output circuit to the input circuit at frequencies removed from the oscillatory frequency by one octave and more, and means for reducing the transmission through said degenerative-coupling means to zero at said predetermined fre quency.

21. An electrical circuit oscillatory at a predetermined frequency and comprising an ampiifier having an input circuit and an output circuit and a single network for'coupling the output circuit regeneratively to the input circuit at the oscillatory frequency and for degeneratively coupling the output circuit to the input circuit at all frequencies removed from the oscillatory frequency by one octave and more, the said network having resistive and capacitive impedances only.

22. An electrical circuit oscillatory at a predetermined frequency and comprising an amplifier having an input circuit and an output circuit and a single network for coupling the output circuit regeneratively to the input circuit at the oscillatory frequency and for degeneratively coupling the output circuit to the input circuit at all freamass quenciesremoved from the oscillatory frequency by one octave and more, the said network having resistive and capacitive impedances only, and means for varying a plurality of the impedances. 23. An oscillator having, in combination, an

amplifier having an input circuit and an output circuit, means for degeneratively coupling the output circuit to the input circuit so as to reduce the effective gain of the amplifier, the said means comprising a three-terminal network having one terminal common to the'input and output circuits and the othertwo terminals respectively connected to the input and output, circuits, means for reducing the transmission through said degenerative-coupling means to a minimum at a predetermined frequency, and means for providing sufficient regenerative coupling between the output circuit and the input circuit to cause selfoscillation to take place at the said predetermined frequency.

24. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, means for degeneratively coupling the output circuit to the input circuit so as to reduce the effective gain of the amplifier, the said means comprising a three-terminal network having one terminal common to the input and output circuits and the other two terminals respectively connected to the input and output circuits, meansfor reducing the transmission through said degenerative-coupling means to zero at a predetermined frequency, and means for providing suilicient regenerative coupling between theoutput circuit and the input circuit to cause selfoscillation to take place at the said predetermined frequency.

25. An electrical circuit, oscillatory at a predetermined frequency and consisting of an amplifier having an input circuit and an output circuit, means for coupling the output circuit regeneratively to the input circuit at the oscillatory frequency, the said means comprising a threeterminal network having one terminal common to the input and output circuits and the other two terminals respectively connected to the input and output circuits, means for degeneratively coupling the output circuit to the input circuit at frequencies removed from the oscillatory frequency by one octave and more, and means for reducing the transmission through said degenerative-coupling means to zero at said predetermined frequency.

26. An oscillator comprising an amplifier having an input circuit and an output circuit, means for degeneratively coupling the output circuit to the input circuit, the said means comprising a three-terminal network having one terminal common to the input and output circuits and the other two terminals respectively connected to the input and output circuits, the said means having substantially minimum coupling at a predetermined frequency and comprising a plurality of resistors and capacitors only, means for varying the resistors only to vary the frequency of the oscillations of the oscillator, and means for providing sufficient regenerative coupling between the output circuit and the input circuit to cause selfoscillation to take place at the said predetermined frequency.

27. An electrical circuit oscillatory at a predetermined frequency and comprising an amplifier having an input circuit and an output circuit and means for coupling the output circuit regeneratively to the input circuit at the oscillatory frequency and for degeneratively coupling 28. An electrical circuit oscillatory at a predetermined frequency and comprising an amplifier having an input circuit and an output circuit and a single three-terminal network for coupling the output circuit regeneratively to the input circuit at the oscillatory frequency and for degeneratively coupling the output circuit to the input circuit at frequencies removed from the oscillatory frequency by one octave and more, the said network having one terminal common to the input and output circuits and the other two terminals respectively connected to the input and output circuits, and means for reducing the transmission through said degenerative-coupling means to zero. at the said predetermined frequency.

29. An electrical circuit oscillatory at a predetermined frequency and comprising an ampli-x fier having an input circuit and an output circuit and a. single three-terminal network for coupling the output circuit regeneratively to the input circuit at the oscillatory frequency and for degeneratively coupling the output circuit to the input circuit at frequencies removed from the oscillatory frequency by one octave and more,.

the said network having one terminal common to the input and output circuits and the other. two terminals respectively connected to the input and output circuits, and the said network having resistive and capacitive impedances only.

30. An electrical circuit oscillatory at a predetermined frequency and comprising an amplifier having an input circuit and an output circuit and a single three-terminal network for coupling the output circuit regeneratively to the input circuit at the oscillatory frequency and for degeneratively coupling the output circuit to the input circuit at frequencies removed from the oscillatory frequency by one octave and more, the said network having one terminal common to the input and output circuits and the other two terminals respectively connected to the input and output circuits, the said network having resistive and capacitive impedances only, and means for varying a plurality of the impedances.

31. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a network having a characteristic curve such that the network shall attenuate a predetermined frequency, means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the input circuit and the voltage applied to the input circuit by the network, and a network for providing suflicient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency.

32. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, a network having an input circuit and an output circuit and having a characteristic curve such that the network shall attenuate a predetermined frequency, and means for degeneratively coupling the output circuit to the input circuit to feed back energy from the output circuit to the input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the input circuit and the voltage applied to the input circuit by the network, and the network comprising means for providing sumcient regenerative coupling between the output circuit and the input circuit to cause self-oscillation to take place at the said predetermined frequency.

33. An oscillator having, in combination, an amplifier having an input circuit and an output circuit, the amplifier comprising a plurality of amplifying stages each having an input circuit and an output crcuit, a network having a characteristic curve such that the network shall attenuate a predetermined frequency, means for degeneratively coupling the amplifier output circuit to the amplifier input circuit to feed back energy from the amplifier output circuit to the amplifier input circuit through the network, the oscillator comprising means for effecting a reversal of phase between the voltage of the amplifier input circuit and the voltage applied to the amplifier input circuit by the network, and means for providing sufficient regenerative coupling in the amplifier so that self-oscillatlon'shall take place at the predetermined frequency.

34. An oscillator having in combination an amplifier having an input circuit and an output circuit, a network having a characteristic such that it attenuates a predetermined frequency,

' means for degeneratively coupling the amplifier output circuit through the network to the amplifier input circuit, the total phase shift through the amplifier and degenerative coupling circuits being such that the application of an alternating voltage to the input circuit of the amplifier from an external source shall result in the application to the same circuit by the feed-back circuit of a voltage of the same frequency but opposing in phase, and means for providing sufilcient regenerative coupling so that self-oscillation shall take place at said'predetermined frequency.

HERMON H. SCOTT. 

